And with 70 percent of all humans due to live in cities by 2050, who will collect their rubbish?

It's easy to get excited about the size of the problem. So why is the cleantech sector so undervalued by investors?

Cleantech investment peaked at US$8.4 billion in 2008 and has not recovered. The listed sector in the US hit a peak of US$475 billion in 2007 but has now slumped to just $142 billion. A recent article on AOL reported that despite the revenues holding up quite well "the multiples investors are willing to pay for those revenues have declined dramatically. Those multiples have compressed by more than 50 percent. The overall S&P has only compressed by 15 percent."

What's the the dirt on cleantech?

Last week I was lucky to hear from a dozen investors and commentators on the state of cleantech at the Cleanvest conference in Tel Aviv.

Here are the top takeouts.

1. Look to corporates: When it comes to sourcing money for start ups and new ideas, go ask a large company. "They're the only ones with any money left," says Gina Domanig, of Emerald Technology Ventures, based in Switzerland. In addition, companies like GE bring customers, credibility and sites for piloting new ideas. Dominig quotes a recent study by GE which shows only 10 percent of investment in clean tech is likely to come from governments; as much as 40 percent will come from corporates.

2. Saving cost is hot: As you'd expect in recessionary times, technologies that lead to cost savings will be more popular than ones that add new features. Paul Nillesen, the clean tech expert for PwC in Amsterdam, says the ideas that are attracting investment now are in data management, monitoring, switching and optimisaton.

3. Avoid subsidised business: In a world where governments are in debt crisis, subsidies and tax breaks are under threat. "So don't go there," says investor Richard Irving of Silicon Valley based Pond Venture Partners. "We avoid anything that relies on subsidies or special government dispensation."

4. Go for the energy/water nexus: "Water is not priced properly yet, but energy is, so we like to go for the water/energy nexus; that is, any ideas that solves problems for both sectors, such as energy savings in water distribution or electricity generation in water treatment."

5. Storage is king: "Anyone that can solve the energy storage problem has the killler app," says David Nahai, the former head of the California water authority, now a consultant. Irving agrees, saying effective storage will be the turnkey for solar and wind energy. "It's the biggest problem, so it's also the biggest opportunity."

6. Give it time: All the speakers emphasised that clean tech has a longer time horizon than ICT. "It takes 20 years," says Irving. Unlike ICT, where an app can be built once and replicated a million times around the world, cleantech is messy, requiring engineering, facilities and customers such as city councils and governments.